cmd/tsconnect: add dev-pkg command for two-sided development

Allows imports of the NPM package added by 1a093ef482
to be replaced with import("http://localhost:9090/pkg/pkg.js"), so that
changes can be made in parallel to both the module and code that uses
it (without any need for NPM publishing or even building of the package).

Updates #5415

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/common.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"net"
 	"os"
 	"os/exec"
 	"path"
@@ -68,6 +69,18 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 	}, nil
 }
 
+func commonPkgSetup(dev bool) (*esbuild.BuildOptions, error) {
+	buildOptions, err := commonSetup(dev)
+	if err != nil {
+		return nil, err
+	}
+	buildOptions.EntryPoints = []string{"src/pkg/pkg.ts", "src/pkg/pkg.css"}
+	buildOptions.Outdir = *pkgDir
+	buildOptions.Format = esbuild.FormatESModule
+	buildOptions.AssetNames = "[name]"
+	return buildOptions, nil
+}
+
 // cleanDir removes files from dirPath, except the ones specified by
 // preserveFiles.
 func cleanDir(dirPath string, preserveFiles ...string) error {
@@ -90,6 +103,27 @@ func cleanDir(dirPath string, preserveFiles ...string) error {
 	return nil
 }
 
+func runEsbuildServe(buildOptions esbuild.BuildOptions) {
+	host, portStr, err := net.SplitHostPort(*addr)
+	if err != nil {
+		log.Fatalf("Cannot parse addr: %v", err)
+	}
+	port, err := strconv.ParseUint(portStr, 10, 16)
+	if err != nil {
+		log.Fatalf("Cannot parse port: %v", err)
+	}
+	result, err := esbuild.Serve(esbuild.ServeOptions{
+		Port:     uint16(port),
+		Host:     host,
+		Servedir: "./",
+	}, buildOptions)
+	if err != nil {
+		log.Fatalf("Cannot start esbuild server: %v", err)
+	}
+	log.Printf("Listening on http://%s:%d\n", result.Host, result.Port)
+	result.Wait()
+}
+
 func runEsbuild(buildOptions esbuild.BuildOptions) esbuild.BuildResult {
 	log.Printf("Running esbuild...\n")
 	result := esbuild.Build(buildOptions)